A quick search of the internet reveals many websites that allow you to download books in PDF format.
The problem with PDFs is that they don’t “reflow”. If you change the size of the browser then you may see just part of the width of the page, rather than the text adjusting to use all the width, and wrapping onto the next line as necessary.
Solid Book is intended to solve this problem by allowing you to view the books in a simple reader.